1. Field of the Invention
This invention relates to improved catalyst compositions and processes for utilizing same. The mixed chromium catalysts are useful for polymerizing olefins and, more particularly, for use in particle form polymerizations for the manufacture of high density polyolefin resins which exhibit high resistance to environmental stress cracking and which are useful for the production of blow molded articles. More specifically, the mixed catalysts are comprised of a first and second chromium-containing supported catalyst component wherein the supports used for said first and second catalyst components have different pore volumes and wherein at least one of the catalyst components contains an additional element selected from aluminum, titanium, zirconium, boron or phosphorus.
2. Description of the Prior Art
In the selection and production of resins for the manufacture of blow molded goods, more so than with other fabrication methods, a careful balance must be struck between the physical properties of the solid resin and the processing characteristics of the resin melt if efficient production of durable molded articles is to be achieved. While many resins have superior physical properties, they do not have acceptable rheological (viscoelastic) properties under conditions of flow and shear such as are encountered during blow molding. Conversely, other resins which exhibit satisfactory viscoelastic behavior are deficient in one or more essential physical characteristics. For this reason there is a continuing effort to develop resins which have an optimal balance of physical and rheological properties. This is especially true with polyethylene resins used for the manufacture of blow molded bottles.
Improved processability of polyolefin resins, i.e., improved flow properties and shear response, is obtained by increasing the molecular weight distribution of polymers. This has been accomplished by blending separately prepared polymers of different molecular weights and by utilizing catalyst systems capable of directly producing polymers having broadened molecular weight distributions. U.S. Pat. Nos. 3,959,178 and 4,025,707, for example, disclose the preparation of ethylene homopolymers and copolymers of broadened molecular weight obtained utilizing a mixed catalyst comprising several portions of the same or different chromium components and metal promoted variations thereof wherein each portion is activated at a different temperature. U.S. Pat. Nos. 4,540,757 and 4,560,733 utilize milled blends of at least two different silica-containing components having different melt index potentials for the preparation of titanium-containing catalysts components.
U.S. Pat. No. 4,263,422 discloses polymerizing .alpha.-olefins utilizing dual independently supported catalysts one of which is an inorganic halide supported titanium and/or vanadium Ziegler type catalyst while the other is an inorganic oxide supported chromium-containing catalyst.
While it has been possible to improve processing characteristics in this manner, any processing advantage has heretofore been offset in large part by a corresponding decrease in one or more essential physical properties. For example, while the products obtained in accordance with U.S. Pat. No. 4,025,707 have good die swell characteristics and acceptable environmental stress crack resistance and flow properties, polymer densities are too low to provide the necessary stiffness for blown bottles. On the other hand, polymers such as those produced using the catalysts of U.S. Pat. No. 4,560,733 have sufficiently high densities (0.960 and higher) but typically are deficient in their resistance to environmental stress cracking.
For blow molding bottles, the resin must have sufficiently high density and high resistance to environmental stress cracking in addition to having acceptable processing characteristics. A density of at least 0.957 is necessary to obtain the high degree of stiffness required by molders. It is considered even more desirable for the resin to have a density of 0.958 to 0.961. Stiffness imparts strength to bottles and also makes thinner wall constructions possible. More units can thus be produced per pound of resin, generally referred to within the industry as "lightweighting," which represents an economic advantage for the processor. Lightweight resins must also exhibit a high degree of resistance to cracking under environmental stress, that is, while being stretched or bent in several directions at once.
While stiffness increases with density as noted above, an inverse relationship exists between density and resistance to environmental stress cracking. As density is increased, resistance to environmental stress cracking is reduced. Both of the above relationships assume that the melt index of the resins are the same or essentially the same. A balance must therefore be struck between the density, i.e., stiffness, of the resin and the resistance of the resin to environmental stress cracking.
It would be advantageous if the molecular weight distribution of polymers could be varied and if polyethylene resins having an optimal balance of rheological and physical properties could be produced. It would be even more desirable if high density resins having high resistance to stress cracking useful for the manufacture of blown bottles could be obtained utilizing mixtures of known catalyst components. These and other advantages are realized using the mixed catalyst compositions of the present invention which are described in detail to follow.